The US coastguard says BP has managed to cap one of three leaks from its stricken deepwater oil well, but the work is not expected to reduce the overall flow of oil into the Gulf of Mexico.

The work should reduce the number of leak points that need to be fixed on the ocean floor, making it easier to drop a containment dome to bottle up the disastrous oil spill threatening sealife and livelihoods along the Gulf Coast.

Since an explosion on 20 April, 50 miles off the coast of Louisiana, the Deepwater Horizon well has been spewing at least 800,000 litres a day.

Experts say the best short-term solution is to drop a specially built giant concrete-and-steel box designed to siphon the oil away over the breach.

Crews for Wild Well Control, a contractor, are putting the finishing touches on the 100-ton containment dome, which is expected to be taken to the leak site today. John Curry, a BP spokesman, said it would be deployed on the seabed by tomorrow.

It's the latest attempt by BP engineers to stem the oil from the rig, which killed 11 workers when it exploded. It sank two days later, and oil started pouring into the Gulf. BP is in charge of the cleanup and President Barack Obamasays the company is responsible for the costs.

Officials said that capping one of the three leaks was a step towards stemming the flow. "It doesn't lessen the flow, it just simplifies the number of leak points they have to address," David Mosley, a coastguard petty officer 1st class, said.

A rainbow sheen of oil has reached land in parts of Louisiana, but the gooey rafts of coagulated crude have yet to come ashore in most places. Forecasts showed the oil wasn't expected to come ashore until at least tomorrow.

"It's a gift of a little bit of time, I'm not resting," said Mary Landry, a US coastguard rear admiral.

In their worst-case scenario, BP executives told members of a congressional committee that up to 9.5m litres a day could spill if the leaks worsened, though it would be more like 6.5m litres.

The worst oil spill in US history resulted from the 1989 grounding of the Exxon Valdez oil tanker in Alaska. It leaked nearly 11m gallons (41 million litres) of crude.

Containment domes have never been tried at this depth, about 5,000ft (1,500m) because of the extreme water pressure. The dome, if all goes well, could be fired up early next week to start funnelling the oil into a tanker.

It was not clear whether the equipment would work, said Bill Salvin, a BP spokesman. "What we do know is that we have done extensive engineering and modelling, and we believe this gives us the best chance to contain the oil, and that's very important to us."

Yesterday seas calmed allowing more conventional methods to contain the spill to get back on track as businesses and residents kept an eye on the ocean currents, wondering when the sheen washing ashore might turn into a heavier coating of oil. Crews put out more containment equipment and repaired some booms damaged in rough weather over the weekend. They also hoped to again try to burn some of the oil on the water's surface.

Chemical dispersants piped 1,500m to the main leak have significantly reduced the amount of oil coming to the surface, BP said. The company also hoped to shut off one of the smaller of three leaks, though it may not reduce the flow very much, said Doug Suttles, BP's chief operating officer.

From the air yesterday, the site of the Deepwater Horizon explosion looked similar to a week ago, except for the appearance of a massive rig brought in to drill a relief well to shut off the spewing oil. However, that would take months.

People along the Gulf Coast have spent weeks living with uncertainty, wondering where and when the oil may come ashore, ruining their beaches and their livelihoods.

The anxiety is so acute that some are seeing and smelling oil where there is none. And even though the dead turtles and jellyfish washing ashore along the Gulf of Mexico are clean, and scientists have yet to determine what killed them, many are sure the flow of crude is the culprit.

The rig was owned by Transocean. Some of the 115 surviving workers aboard when it exploded are suing the company and BP. In lawsuits filed yesterday, three workers say they were kept floating at sea for more than 10 hours, while the rig burned uncontrollably. They are seeking damages.

Guy Cantwell, a spokesman for Transocean, defended the company's response, saying 115 workers got off alive. Two wrongful death suits have also been filed.

While officials worked on the cleanup, the long wait took its toll on the surrounding coastline.

Perdido Key, a Florida barrier island between Pensacola and the Alabama state lined with sugar-white sand and studded with condominiums, could be the first place to be affected by the oil spill.

The air cleared later, but Owensby's 28-year-old daughter, Stephanie, who tends a bar at the lounge, said some visitors had complained of feeling ill from the fumes.

"It's very sad because I grew up out here," she said. "I remember growing up seeing the white beaches my whole life. Every day I've been going to the beach … a lot of people are out watching and crying."